1. Field of the Invention
The present invention relates to optical pickups capable of adopting disc type recording media having different formats, and more particularly, to a compatible optical pickup for a compact disc rewritable (CD-RW), a member of the CD family media, and a digital video disc read only memory (DVD-ROM).
2. Description of the Related Art
Recently, the need for an optical pickup capable of recording and/or reproducing information on and/or from a DVD at a high density be compatible with CD family media such as CD, recordable CD (CD-R), CD-RW, CD interactive (CD-I) and CD plus graphics (CD+G) has arisen.
The standard thickness of existing CD family media is 1.2 mm, whereas the thickness of DVDs has been standardized to 0.6 mm in consideration of the allowable error in the tilt angle of a disc and the numerical aperture (NA) of an objective lens. Accordingly, when recording or reproducing information on or from a CD using an optical pickup for DVDs, spherical aberrations occur due to a difference in the thicknesses therebetween. Such spherical aberration cannot provide a light intensity sufficient for recording an information (radio frequency) signal or may deteriorate the signal reproduced from the CD. Also, DVDs and CD family media utilize different wavelengths of light for reproduction: CDs use light having a wavelength of about 780 nm as a light source for reproduction, whereas DVDs use light having a wavelength of about 650 nm as a light source therefor. Thus, for the compatibility with CDs, an optical pickup adopting a light source capable of emitting different wavelengths of light, and a structure in which optical spots can be formed at different focal positions is required.
Referring to FIG. 1, a conventional compatible optical pickup comprises a first light source 21 for emitting light having a wavelength of about 650 nm and a second light source 31 for emitting light having a wavelength of about 780 nm. The first light source 21 is appropriate for a DVD 10a, which is a relatively thin disc, and the second light source 31 is appropriate for a CD 10b, which is a relatively thick disc. The first and second light sources 21 and 31 are selectively driven according to the type of adopted optical disc, a DVD or a CD.
Light emitted from the first light source 21 is condensed by a first collimator lens 23, parallel incident onto a first polarization beam splitter (PBS) 25, and then reflected by the first PBS 25 toward the DVD 10a. After reflected by the DVD 10a, the reflected light is transmitted through the first PBS 25 and is then received by a first photodetector 27. Here, an interference filter 41 for changing the paths of light emitted from the first and second light sources 21 and 31, a xc2xc-wavelength plate 43 for delaying the phase of light incident thereto to change the polarization characteristic, a variable diaphragm 45, and an objective lens 47 for condensing light incident thereto are situated on an optical path between the first PBS 25 and the disc 10.
Light emitted from the second light source 31 is condensed by a second collimator lens 33, parallel incident onto a second PBS 35, transmitted through a condenser lens 37, and then incident onto the interference filter 41. The light is reflected by the interference filter 41 and passes through the xc2xc-wavelength plate 43, the variable diaphragm 45 and the objective lens 47 in sequence to form an optical spot on the CD 10b. 
Light reflected by the CD 10b is incident onto the interference filter 41 through the objective lens 47, the variable diaphragm 45 and the xc2xc-wavelength plate 43, and then reflected by the interference filter 41 heading toward the second PBS 35. The light is reflected by the second PBS 35 and received by a second photodetector 39.
The interference filter 41, an optical element for transmitting or reflecting incident light depending on the wavelength of the incident light, transmits the light emitted from the first light source 21 and reflects the light emitted from the second light source 31.
Referring to FIG. 2, the variable diaphragm 45 comprises a first region 45a, which is variable in size, and a second region 45b bounding the first region 45a, and selectively transmits the light incident onto a region of the objective lens 27 having a NA of 0.6 or less. The first region 45a, which corresponds to a region of the objective lens 47 having a NA of 0.45 or less, completely transmits the light emitted from the first and second light sources 21 and 31. The second region 45b, which corresponds to a region of the objective lens having a NA of 0.45 to 0.6, completely transmits light having a wavelength of 650 nm and completely reflects light having a wavelength of 780 nm.
In the conventional compatible optical pickup, the variable diaphragm completely reflects the light for recording on a CD-R, which is emitted from the second light source and incident onto a region having a NA of 0.45 or more, and thus it cannot be applied to an optical pickup for CD-RWs, which requires a NA of 0.5 or more and high optical efficiency for recording. Also, the need for the variable diaphragm having first and second regions, which is manufactured through sophisticated and expensive processes makes assembling of such optical pickup complicates and increases the manufacturing cost.
It is an object of the present invention is to provide a compatible optical pickup capable of reproducing information from a digital video disc read only memory (DVD-ROM), in addition to writing or reproducing information on or from a compact disc rewritables (CD-RWs), which ensures output of a wavelength of 780 nm with a high optical power, and a high numerical aperture (NA) of approximately 0.5 or more.
To achieve the above object, the present invention provides a compatible optical pickup comprising: a first light source for emitting a first light having a predetermined wavelength; a second light source for emitting a second light having a long wavelength relative to the first light; an optical path changing means for changing the traveling path of an incident light; an objective lens for condensing the first and second lights onto optical discs having different thicknesses, respectively; and a main photodetector for detecting an information signal and error signals from the first and second lights which have been reflected by the optical discs and passed through the optical path changing means, wherein the optical path changing means includes a planar first beam splitter disposed on the optical path between the first light source and the objective lens, for changing the traveling path of the first light from the first light source, and a planar second beam splitter disposed on the optical path between the first beam splitter and the objective lens, for changing the optical paths of the first and second lights, and the main photodetector comprises a first photodetector having four divided regions, a second photodetector having two divided regions, and a third photodetector having two divided regions, the second and third photodetectors being disposed at both sides of the first photodetector.